The present invention relates generally to AC relays and more particularly to AC electromagnetic relays of the type which are closed when deenergized and opened as the AC current ramps upwardly to a certain level and for example are operated by a solar photocell to automatically switch one or more outdoor lights off and on (by opening the relay when the solar light increases to a certain intensity and reclosing the relay when the solar light decreases to a certain intensity).
Photocell operated relays are commonly used for controlling street lights. To minimize the total system cost, a separate AC relay is customarily provided for each street light. Also, cost and other considerations dictate that a relay with a normally closed relay switch be used and the relay be connected to turn the light on when closed and off when open. Such an arrangement ensures that the light will be on when needed and permits using the light, if on in the daytime, to indicate a malfunction. Typically, the photocell is connected in series with the relay so that the AC voltage across the relay is dependent on the photocell resistance and thus the intensity of the light received by the photocell. As the light intensity increases, the relay current increases or ramps upwardly. As the light intensity decreases, the relay current decreases or ramps downwardly. The relay is opened when the relay current increases to a certain level and recloses when the relay current decreases to a certain lower level.
When a conventional, normally closed AC relay is operated by an upwardly ramping AC current as described, just before the AC current reaches the required level to open the relay, the relay armature can vibrate sufficiently to cause the mating contacts of the normally closed relay switch to chatter. Such incipient vibration and switch chatter is caused by the varying magnetic field across the working air gap of the relay and the resulting ripple actuating force. In a solar photocell controlled system, such incipient vibration and switch chatter can occur for a significant period of time due to the gradual increase in the solar light intensity to the level required to open the relay. Such incipient switch chatter can significantly reduce the operating life of the relay and outdoor light or other electrical device controlled by the relay. Some incipient switch chatter is permitted under the standards of acceptance of the OLC ("Outdoor Lighting Controls") industry--i.e., three contact breaks and restrikes are permitted within a period of fifty milliseconds. However, accepting that level of switch chatter is tantamount to accepting a reduced operating life of both the relay and outdoor light.
Attempts to prevent or minimize incipient switch chatter have included using nickel-iron alloys for all or part of the ferromagnetic structure of the relay to flatten the permeability curve of the ferromagnetic structure, in relationship to the relay current, at the current level of incipient switch break. However, nickel-iron alloy parts are very expensive in comparison to conventional soft iron parts due to both the higher cost of the material and the protracted annealing process required. Accordingly, nickel-iron alloy parts are typically made with a minimum of material and consequently are relatively weak structurally and highly sensitive magnetically. Such relay parts are therefore vulnerable to physical damage and loss of calibration, particularly where, as in some commercial relays, the working air gap is only a few thousandths of an inch.